There is some
talk that lowering
an NVIS or N.V.I.S.
antenna very close
to the ground
enhances NVIS
operation. The
problem is very few
people have actual
measurements. They
either use computer
models (mostly
NEC-2) or
seat-of-the-pants
feelings about what
happens.

I've decided to
post some actual
measured data from a
Viet Nam war study
of NVIS antennas.
The little circles
that appear above or
below the smooth
curves are actual
field strength
measurements made in
1970 by Hagn and
Barker in Thailand.

Compare the
circles to NEC-3
curves (the curved
lines) for two
different ground
conductivities in
the model. This was
an attempt to match
real-world measured
data using accurate
test equipment and
helicopters to
measure FS above a
real dipole with a
model on NEC at
5MHz.

This data is over
very good SE Asia
rich black jungle
soil, much better
than found in most
US locations.
This means ground
losses are
lower than
we would have in
most locations. I'm
sorry graph quality
is so poor, but the
paper is a very old
thermal paper image.
I'm happy any of the
pages are still
readable.

Notice between
.02 wavelength and
.12 wavelengths the
measured data
(circles) matches
the computer model
closely. At about
.02WL the
conductivity in the
model had to be
decreased to match
the actual data.
Peak field strength
was +8dBi at 24 feet
(measured at 5MHz).

MEASURED DATA

At .005
wavelength (1 foot
high 5MHz) the field
strength was -17dBi,
down 25dB from
0.12 wavelength
height.

At .02 wavelength
(4 feet high at
5MHz) the field
strength was -5dBi,
down 13dB from 0.12
wavelength height.

At .04 wavelength
(8 feet high at
5MHz) the field
strength was 3dBi,
down 5dB from 0.12
wavelength height.

At .06
wavelengths high (12
feet at 5MHz) field
strength is down 3dB
from 0.12 wavelength
high.

To convert these
points to other
frequencies multiply
the height in
wavelength by the
wavelength in feet.
Use F / 984 =
wavelength in feet.

For example
3.8MHz would be 984
divided by 3.8 = 259
feet. This means a
.02 wave high
antenna (13 dB of
power loss) would be
259 times .02 = 5.2
feet high.

We can reasonably
expect, over very
good soil, a 5.2
foot high antenna
would be down 13dB
from an antenna 31
feet high. This
would be true at any
distance.

EZNEC MODELS

Height .12
wavelength (31 feet)

Compare this to
+8dBi measured in
Thailand. Very good
agreement. Now let's
move the antenna
lower.

Height .02
wavelength
(5.2 feet)

Compare this to
-5dBi for the real
antenna and we see
it is still very
good agreement. This
shows both the
EZNEC+ model and a
real measurement
agree closely.
EZNEC+ tells us we
lost 11.6dB while
the actual
measurements tell us
we lost 13dB.

Height .005
wavelength (1.3
feet)

We now see EZNEC+
shows -11.4dBi which
is
-19.4dB from our 31
foot high antenna.
The actual
measurement from
Thailand shows the
signal was down
25dB from the higher dipole. In this case EZNEC+ is
overestimating
signal strength by
about 6dB.

Now here is
something to
consider. People
claim distant
signals get weaker
and short skip
signals get stronger
when the antenna is
lowered to very low
heights. Factually
that just isn't
true. We have
measurements made
with sophisticated
equipment using
helicopters to fly
over the antenna
area and plot
radiation. Those
measurements prove
we lose
considerable signal
level on
transmitting
as the antenna is
lowered below .05
wavelength (about 12
feet). We have
models that closely
agree (except at
very low heights).

We also see the
pattern hardly
changes. If the
pattern or
directivity hardly
changes, the
signal-to-noise
ratio on receive
will not change
either. The only
difference will be a
weaker signal and
weaker noise, just
like adding an
attenuator. The
problem is a low
antenna kills your
transmitting signal,
turning a 100W rig
into the equivalent
of a 1 watt
transmitter in the
case of a very low
antenna.

I'm afraid I have
to side with R. Dean
Straw's QST article,
Hagn-Barker's actual
field strength
measurements,
EZNEC+, and my own
experience measuring
and using antennas
for the past 40+
years.

Please, let's not
give silly advice
like 5-foot high
antennas are good
ideas for emergency
communications or
NVIS operation. Very
low antennas produce
very low signal
levels at any
distance when
compared to antennas
of modest height.

Ground Screens below NVIS Antennas

Some claim a screen, counterpoise, or reflector wire grid below a low dipole
will not improve signal levels. My own direct experience shows the idea a screen
will not improve signal levels is NOT true, and that a counterpoise system can
actually improve signal levels. This is true even when the screen or
counterpoise is not tuned. On 75-meters, back in the days of 3830 and 3895 radio
wars, there was always a definite advantage to stations using low dipole with
good screens or counterpoises below the antenna. this was a trick I first earned
from W8PSX in Westlake Ohio , where Jim had a counterpoise system below his
160-meter dipole.

Let's look at losses in a low 80-meter dipole over soil like mine:

This antenna with no ground screen has 4.81 dBi gain

One counterpoise wire below antenna.

This antenna has almost 2 dB more signal level.

This antenna, with three untuned ground counterpoise wires, has 3
dB more signal level.

With many counterpoise wires, the model improves 3.7 dB.

It's my belief the actual improvement I saw was around 5 dB in my
own systems, although this varies greatly with antenna height and soil
conductivity.